The present invention relates to the field of mass-storage devices. More particularly, this invention relates to a method and circuit for generating low-noise current for magneto-resistive heads.
One key component of any computer system is a device to store data. Computer systems have many different devices where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations over the disc, and circuitry that is used to write and/or read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved from and written to the disc surface. A microprocessor controls most of the operations of the disc drive, in addition to passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The disc drive includes a transducer head for writing data onto circular or spiral tracks in a magnetic layer the disc surfaces and for reading the data from the magnetic layer. In some drives, the transducer includes an electrically driven coil (or xe2x80x9cwrite headxe2x80x9d) that provides a magnetic field for writing data, and a magneto-resistive (MR) element (or xe2x80x9cread headxe2x80x9d) that detects changes in the magnetic field along the tracks for reading data.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc-drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of a disc drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
Typically, a current-source circuit is used to provide a constant bias current for the MR element in the read head. Changes in the resistance of the MR element result in a change in voltage across the MR element, and a voltage-detector circuit is used to detect those changes. Any noise in the current source will result in noise in the voltage across the MR element, and thus in the detected voltage.
There is, therefore, a need for a method and circuit for generating low-noise current for transducers, and in particular for magneto-resistive sensing elements.
A method and apparatus is described for generating low-noise current for a disc-drive head, and in one embodiment, for a magneto-resistive head.
One embodiment provides a current-generating circuit, a disc-drive system using the circuit, and a related method. The system includes a rotating disc, a transducer having a read head positioned to read data from the disc, and the current-generator circuit. The current-generator circuit is operatively coupled to the read head. The current-generator circuit includes a field-effect-transistor current-mirror circuit and an automatic adjustment circuit operatively coupled to the current-mirror circuit to adjust an effective width-to-length ratio of field-effect-transistors of the current-mirror circuit in order to reduce noise. In one embodiment, the adjustment is made automatically until a minimum acceptable voltage is obtained across a current-setting sink circuit.
Another aspect of the present invention provides a current-generator circuit for generating low-noise current for a transducer or other application. This circuit includes a field-effect-transistor current-mirror circuit and an automatic adjustment circuit operatively coupled to the current-mirror circuit that adjusts an effective width-to-length ratio of field-effect-transistors of the current-mirror circuit in order to reduce noise.
Another aspect of the present invention provides a method for reducing device noise in a current mirror circuit. The method includes the steps of (a) providing a plurality of current-mirror pairs of field-effect transistors, and (b) selectively disabling one or more of the pairs of field-effect transistors to adjust a width-to-length ratio and thus to reduce device noise.
Another aspect of the present invention provides a disc drive system that includes a rotating disc, a transducer having a read head positioned to read data from the disc, and a current-generator means for automatically adjusting a width-to-length ratio in order to reduce noise.
Advantageously, the system, circuit, and method described for generating low-noise current for a transducer can be activated as part of the disc-drive power-on sequence (and optionally at other times as well) to automatically adjust the effective width-to-length ratio of a transducer bias current source. In some embodiments, the width-to-length ratio is adjusted by selectively disabling one or more pairs of field-effect-transistors that are wired in parallel as current-mirror current sources, until the smallest width-to-length ratio is achieved that will still maintain at least a minimum desired voltage across a current sink. In some embodiments, the current generated for the transducer is a multiple, such as ten, times the current passed to the current sink.